Method and apparatus for grinding material to a fine degree



R. T. HUKKl 3,056,561

ING MATERIAL TO A FINE DEGREE Oct. 2, 1962 METHOD AND APPARATUS FORGRIND 2 Sheets-Sheet 1 Filed Dec. 8, 1959 Q/sro 7:4PA N/ /ggg/ WQKQ,

ATTORNEFY R. 'r. HUKKI 3,05

me MATERIAL TO A FINE DEGREE Oct. 2, 1962 METHOD AND APPARATUS FOR GRIND2 Sheets-Sheet 2 Filed Dec. 8, 1959 PA /v/ HUKK/ INVENTOR.

BYWZFQQW A T TOR/V5 X 3,ll56,561 METHGH) AND APPARATUS FOR GRINDINGMATERHAL TO A FINE DEGREE Risto Tapani Hulrlri, (ltakallio, Otaniemi,Finland, as-

signor to llnsinooritormisto-Engineering Bureau R. T. Hulklri,Helsingfors, Finland, a corporation of Finland Filed Dec. 8, 1959, Ser.No. 858,134 Claims priority, application Sweden, Dec. 13, 1958 6 Claims.(Cl. 241-26) The present invention relates generally to the grind ing orpulverizing art and has particular relation to an improved method andapparatus for grinding material to a high degree of fineness andutilizing a mill of the type having a drum rotating about a horizontalaxis and containing a tumbling load consisting of the material to beground and of grinding media.

The method and inventive organization are applicable for grindingmineral raw materials such as they occur in nature, industrial productssuch as cement clinker and organic materials such as wood fibres andseeds, to the degree of fineness necessary for further treatment orutilization of the said materials, and to illustrate the materials thatmay be employed with and the fineness of the grind obtainable with theinvention it may be mentioned the ores for flotation processes areground to a fineness of about 9099% through a 65 mesh sieve (208microns), and the raw material in cement-making is ground to a finenessof at least 80-90% through a 200 mesh sieve (74 microns), while cementclinker is ground to the fineness of about 80-90% through a 325 meshsieve (44 microns).

it is well known that in order to effectively and emciently grindmaterial to a fine degree the grinding action must be based on friction.For example, a grinding member moved relative to a stationary supportwill subject any material of suitable initial fineness which is placedbetween these two elements to an effective pulverization, based onfriction, with the fine grinding capacity of such a system beingprimarily dependent on the load of the grinding member and the speeddifference in the friction zone between said member and the support.

In mills of conventional design employing a mill or grinding drum whichrotates about a substantially horizontal axis a similar effect isobtained. The charge of grinding bodies corresponds to the grindingmember referred to above and the mill lining corresponds to the support.The outer layer of the charge and the mill lining will rotate in thesame direction, but the lining rotates at a greater angular velocitythan the outer layers of the grinding charge so that a sliding occurswhich brings about the grinding action at the location of the sliding,i.e. the so-called friction or attrition zone.

The resulting difference in velocity in the friction zone will, in fact,be substantial only if the speed of the mill, i.e. the grinding surfaceor lining, exceeds the so-called critical speed. The critical speed fora mill is the speed at which the force of gravity due to the weight of agrinding ball equals the centrifugal force required to maintain the sameball in a circular motion at the inner periphery of the mill. Amathematical expression for the critical speed is where n is measured inrevolutions per minute, and D is the inside diameter of the mill infeet. While the mill proper is rotating at supercritical speed, i.e. aspeed above the critical, the angular velocity of the outer layers ofthe grindin medium must correspond to a sub-critical speed, i.e. a speedbelow the critical speed. For further dis i gt? cussion on this subjectreference is made to applicants co-pending application Serial #729,115filed April 17, 1958, now Patent No. 2,991,617 and entitled GrindingMethod in Ball, Tube and other Mills.

Grinding of a material by means of itself, i.e. the socalled autogenousgrinding, offers great advantages, due primarily to the low cost of thegrinding bodies. When autogenous grinding is involved it is customaryinstead of speaking of mill charge and grinding bodies to refer to thetumbling load as comprising the material to be ground and the grindingbodies which are usually, in fact, bigger lumps of the material proper.

in all cases where autogenous fine grinding has been adopted inpractice, either in connection with subcritical or with supercriticalmill speeds, it has been found that the relatively low specific gravityof the grinding bodies involves a heavy reduction in the grindingcapacity of the mill compared with a corresponding grinding operationcarried out with the use of metallic grinding media such as steel ballsor cylinders.

It has now been found that in grinding by means of rotating drum millsthe two most decisive limiting factors for fine grinding in general andautogenous grinding in particular, i.e. the specific gravity of thetumbling load and the speed of the mill, can be avoided by the provisionof special means which on the one hand indirectly increases the activemass of the tumbling load and on the other hand increases the speeddifference between the outer layer of the tumbling load and the milllining. In this manner the grinding capacity of the mill will beapproximately unaifected by the specific gravity of the tumbling load.Furthermore, the maximum mill speed that can be used will be equallyindependent of the specific gravity of the said load. Consequently itwill be possible to use speeds higher than those ever used in practicebefore. However, a desired speed difference can be provided at aconsiderably lower mill speed than hitherto and so the fine grindingcapacity of a rotating mill according to the invention can be verysubstantially improved, even increased several times as compared withthe capacity of corresponding mills of conventional design.

According to the invention these aims are obtained by the provision ofmeans inside the mill capable of producing a braking effect upon themovement of the tumbling load in relation to the mill drum so as toincrease the pressure exerted by the tumbling load against the millshell beneath the said means.

The mill drum is advantageously rotated at constant, preferably criticalor supercritical speed.

A mill for carrying through the method of the invention is equipped witha special device, referred to hereinafter as a brake member or simply abrake, and arranged so as to produce the desired braking effect upon themovement of the tumbling load. While the mill is rotating atsubstantially uniform speed in the desired direction, the brakeincreases the active mass in the mill by a desired active mass,simultaneously, wholly or partly, reducing to the desired extent theangular velocity of a certain outer layer of the tumbling load relativeto the inner surface or periphery of the mill drum. As a result, afriction grinding zone will be created between the retarded or stoppedlayers of the tumbling load and the rotating mill lining. The milllining may be a lining made wholly or partially of metal or be asubstantially autogenous lining separated from the charge in the mill byseparate retarding means, so-called holding bars, provided at the millshell or on a protecting shell lining and also, if desired, on a liningon the inner side of the mill ends.

For special purposes the lining on the .mill shell may consist ofrelatively wide sections of a metallic liner having a relatively uniformsurface, alternating with an autogenous liner having a somewhat coarsersurface and preferably equipped with holding bars.

The tumbling load is introduced into the mill in conventional manner, inbatches or continuously.

It will be appreciated that autogenous fine grinding in thesecircumstances will take place under conditions resembling those referredto previously, i.e. a stationary autogenous grinding member (thetumbling load and the brake) will be dragged relative to a movingsupport (the mill lining) with a certain amount of material (part of thetumbling load) being introduced between the two members. The unavoidableand even heavy wear on the tumbling load and the lining will now bedirected almost exclusively to the material to be ground by itself withresulting further improved quality of the final product.

The brake may be designed in a number of ways. lts suspension axis maybe coinciding with the axis of the mill or it may be arrangedeccentrically in relation to the latter. The brake may be rigidlysuspended or movably suspended in such manner that it can freely adoptits angular equilibrium within the mill. Furthermore it may be arrangedso as to be positively moved to and fro from an intermediate position,such movements resulting in regular or irregular cyclic variations inthe difference between the velocity of the tumbling load and that of thelining.

The brake preferably consists of a substantially horizontal suspensionshaft supporting one or more separately independent or interconnectedbraking members such as a hammer, rails, rods, strips, rings, chains orany other type Whatever weighing from one to several hundred kilogramseach. Outside the mill the suspension shaft may be provided with acounterweight in order to produce the desired torque inside the mill andin order to facilitate manipulations and repairs the brake is preferablymanufactured as a separate unit which can be mounted or movedindependent of the mill drum as by means of a crane. Normally, the brakeproper is introduced through the outlet opening of the mill, but itssupporting device may more conveniently be placed outside the mill.

Although the principal amount of Wear is concentrated on the tumblingload and on the lining, there will, of course, also be a certain amountof wear on the brake and the parts of the brake which are subject towear are made of a wear resistant material with the material varyingfrom scrap iron to cast iron, hardened carbon steel, cast manganesesteel, a large number of alloy steels resistant to wear, sinteredtungsten and vanadium carbides, alloys of cobalt, alloys of chrome andalloys of tungsten such as stellites etc. The braking member proper maybe made solely of such materials or may be coated with wear-resistantsections prepared beforehand.

If a substantially autogenous lining is used, the wear on the liningwill be concentrated on the actual material to be ground. In that casethe retarding means necessary for creating the autogenous lining, i.e.the holding bars, are made of one of the previously named wear resistantmaterials.

The mill or mill drum used for carrying the invention into effect may beof substantially conventional design. If, however, the mill is usedspecifically for -autogenous grinding, a substantial economy isobtainable if the mill is supported and rotated by rollers covered by aheavy layer of rubber or having pneumatic tires and which engage theouter surface of the drum. By this method all the normally mostexpensive parts such as conventional supports and gear 'wheels aredispensed with. Furthermore, the reduced weight of the autogenousgrinding medium, the reduced quantity of the medium and the mechanicallymore advantageous supporting of the mill from below instead of at theends permits a reduction in the required strength of the mill drum andconsequently in its weight unloaded. The mill speed adopted when usingthe present invention may be sub-critical, critical or supercritical inaccordance with the definitions given hereinbefore.

The present invention can be utilized in connection with wet and drygrinding performed in an open or closed circuit and in combination withany outer screening or concentrating device whatever. However, inconnection with wet grinding the conventional classification effectedoutside the mill may be eliminated because an effective classifyingbasin is produced within the mill proper by a suitable design of theoutlet end. Such a classifying basin is created in wet grinding mills onan industrial scale when operating in accordance with the presentinvention and wherein the discharge of the material takes place at apoint approximately 30 cm. or more above the lowest location of theeffective lining of the mill. Since the cataracting and cascading effectof the tumbling load under the circumstances described is prevented bythe brake, to a large extent, excellent conditions for preventingsedimentation will arise. However, as the coarser parts of the solidmaterial contained in the mass deposit as sediments on the bottom of thebasin, a considerable amount of the material will be introduced chieflyin the friction zone.

Accordingly it is an object of this invention to provide and improvedmethod and apparatus for grinding or pulverizing materials to a highdegree of fineness.

A further object is to effect this improvement in the pulverizing art ina relatively simple and expeditious manner utilizing a horizontallydisposed mill drum rotating about its principal, horizontal axis.

A still further object is to provide such an improved method andapparatus which is extremely efficient in operation providing asubstantially increased capacity over that heretofore obtained in thetype of mill to which the invention pertains while at the same timemaintaining Wear and operating problems at a minimum.

Further and more detailed objects will in part be obvious and in part bepointed out as the description of the invention, taken in conjunctionwith the accompanying drawing proceeds.

In that drawing:

FIG. 1 is a vertical, transverse, sectional view of a drum in accordancewith the preferred embodiment of the invention, with this view depictinga wet grinding mill and;

FIG. 2. is a longitudinal vertical sectional view taken generally alongline 2-2 of FEGURE 1.

Referring now to the illustrative organization of the drawing thesubstantially cylindrical mill or mill drum 1 as depicted therein issupported and rotated by means of two pairs of rollers 2 provided withpneumatic tires 3. The mill hell is provided with live rings 4 thatengage these tires and one or both pairs of rollers 2 may be rotateddirectly or indirectly by any suitable driving means. In order to limitlateral movement of the mill vertically mounted rollers 5 are preferablyprovided adjacent each end of the mill for engagement with the mill endfaces.

The illustrative mill is of the type having an autogenous lining so thatimmediately inside of the cylindrical shell are secured thelongitudinally extending holding bars 8 while radially disposed bars 10extend along the inner surfaces of the end faces and effectively formcontinuations of the bars 8.

The tumbling load comprising the material to be ground, the autogeneousgrinding media or grinding bodies and water is continuously introducedinto the mill through feeding trough 6. A part of the tumbling loadconstitutes, with the aid of the holding bars 8, the autogenous shelllining 7, whereas another part of the tumbling load constitutesautogenous grinding members 11 and still another part of th said loadconstitutes a mass basin 12 from the upper part of which a continuousdischarge through an outlet opening 13 is taken. The autogenous liningat the end wall of the drum is shown at 9.

In order to retard the mo vement of the tumbling relative to the mill,as mentioned previously, a brake organization, designated generally 1 ismounted for disposition within the mill with this brake beingillustratively disclosed as in eccentric relation to the axis ofrotation of the mill with the brake being laterally displaced fro-m theaxis of the mill drum in the direction of rotation relative to the lowerregion of the drum. This brake consists of a suspension shaft 15 whichcarries six uniform hammers 16 that extend downward therefrom into thetumbling load with each hammer consisting of a handle 17 and a head 18fastened to the end thereof, with these heads preferably being made ofwear-resistant steel alloys. The brake organization 14 is supported soas to be angularly displaceable and for this purpose the outer end ofshaft 15 is round and is mounted in a bearing 19 so as to be movableabout its axis. This bearing 19 is secured to and sup orted by asuitable support bracket Zti. Mounted on the outer end of the shaft 15is a regulative weight 21 by means of which the torque inside the millcan be varied i.e. the torque that the brake member exerts against thetumbling load. The weight 21 is only diagrammatically shown and it willbe understood that either the magnitude of the weight or its connectionwith the end of shaft 15 may be varied to vary its reaction upon theshaft.

The eccentric or off-center mounting of shaft 15 causes the hammers tomove closer to the grinding surface of the mill drum as they are rotatedin the counterclockwise direction as viewed in FIG. 1 with this beingthe direction of rotation of the mill drum. The hammers cause anincreased retarding action on the tumbling load as they move closergiving a somewhat automatic operation increasing their effectivenesswhen the tumbling load is such that it tends with greater force to movewith the rotation of the drum.

To facilitate the introduction and removal or regulation of the brake inhorizontal, vertical or angular direction special mechanical devices orhydraulic systems may be provided on the foundation 20.

As shown in FIG. 1 a friction zone 22 will be established between themovable lining 7 and the grinding member 11. The grinding media passingthrough the space between the hammer heads 13 and the mill lining 7 willbe circulated until is has been reduced to the fineness of th finalproduct. In the embodiment illustrated the speed of the mill shellcorresponds to a supercritical speed and as a result the autogenouslining is retained by means of the centrifugal force in its position inrelation to the mill shell even in its uppermost position.

In a dry grinding mill the method is substantially the same as theillustrative wet mill although special provision may have to be made tocheck the formation of dust.

It will be understood that in lieu of the mill being cylindrical it maybe conical or be made up of cylindrical and conical sections and insteadof being mounted on rollers as illustratively disclosed it may besupported other ways, e.g. by means of end trunnions of slide shoebearings as used in conventional mills and it may be driven by othermechanisms such as gear wheels of convenient design.

Though the invention has been shown and described from the standpoint ofcertain structural details in which it is embodied, it is not however tobe considered as being limited by such showing. Rather, as will beapparent to those skilled in the art, certain modifications orvariations may be made in the structure as shown and described, withoutdeparting from the spirit or scope of the invention. It is, accordingly,intended that all matter contained in the above description and shown inthe accompanying drawing shall be interpreted as illustrative and not ina limiting sense.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. The improved method of grinding material to a fine degree comprisingintroducing a tumbling load including material to be ground and grindingmedia, into a horizontally disposed mill drum, rotating the drum aboutits horizontal axis at a predetermined generally constant supercriticalspeed, mechanically retarding the movement of the tumbling load relativeto the drum by introducing a retarding element part way into thetumbling load from the inner border thereof to a position spacedinwardly from the inner surface of the drum for a limited portion of theangular extent of the tumbling load and somewhat forward of thelowermost drum region relative to the direction of rotation of the drumthereby retarding the cascading and cataracting effects and promotinggrinding action due to the attrition effect reducing the angularvelocity of the tumbling load at the location relative to the innersurface of the drum, increasing the pressure exerted by the tumblingload against the mill shell and increasing the friction action betweendifferent portions of the material to be ground and between the materialto be ground and the grinding media.

2. The improved method of grinding material as in claim 1 and including,providing an autogenous grinding surface on said drum and effectinggrinding between said tumbling load and said autogenous grindingsurface.

3. In a grinding mill of the type described, a horizontally disposedgrinding drum into which a tumbling load is fed and from which finelyground material is dis charged, means for rotating said drum about itshorizontal axis at supercritical speed, arm means extending from aposition in said drum outside of said tumbling load down into saidtumbling load generally throughout the length of the drum and said armmeans terminating in substantial spaced relationship with respect to theinner surface of said drum, said arm means, adjacent the terminatingportion thereof, being provided with wear resistant members on the sideof said arm means opposed to the direction of movement of said drum,said arm means being of limited extension with respect to the directionof rotation of said drum so as to act as a brake to retard movement ofthe tumbling load resulting from the rotation of the drum, means formounting said arm means in the drum and tending to maintain said armmeans generally stationary while said drum rotates.

4. In a grinding mill of the type described, a horizontally disposedgrinding drum into which a tumbling load is fed and from which finelyground material is discharged, means for rotating said drum about itshorizontal axis at supercritical speed, arm means extending from aposition in said drum outside of said tumbling load down into saidttunbling load generally throughout the length of the drum and said armmeans terminating in substantial spaced relationship with respect to theinner surface of said drum, said arm means, adjacent the terminatingportion thereof, being provided with wear resistant members on the sideof said arm means opposed to the direction of movement of said drum,said arrn means being of limited extension with respect to the directionof rotation of said drum so as to act as a brake to retard movement ofthe tumbling load resulting from the rotation of the drum, and a supportfor said arm means, said support including yieldable restraining meansfor restraining said arm means from movement by the action of the movingtumbling load thereon.

5. In a grinding mill of the type described, a horizontally disposedgrinding drum into which a tumbling load is fed and from which finelyground material is discharged, means for rotating said drum about itshorizontal axis at supercritical speed, a support shaft extending intothe mill drum longitudinally thereof and generally throughout the lengththereof, a plurality of arms carried by said shaft and extendinglaterally of said shaft from a position outside of the tumbling loadwithin said drum, down into said tumbling load, said arms terminating insubstantial spaced relationship with respect to the interior of saiddrum, said arms being of limited extension References Cited in the fileof this patent with respect to the direction of rotation of said arms soUNITED STATES PATENTS as to act as brakes to retard movement of saidload as a result of rotation of the drum and at the location of said478,252 culllngwofth y 5, 1392 arms and accordingly increase thepressure exerted by the 5 639,405 Krelss 1899 tumbling load on the innersurface of the drum, enlarged 926441 Shaffer June 29, 1909 wearresistant portions carried by said arms adjacent the 2,282,887 Rober'tsy 1942 terminating portions thereof on the side thereof opposed to thedirection of rotation of said drum and means yield- FOREIGN PATENTS ablyrestraining said shaft and arms from rotating with 10 1,013,015 FranceApr. 23, 1952 and in response to the movement of said tumbling loadresulting from rotation of said drum. OTHER REFERENCES 111 a grindingmill as in Claim 5, Said Shaft being Hukki: Mining Engineering, May1958, pages 581-591.

parallel with but laterally offset With respect to the axis of rotationof said drum, said ofiset being in the direction 15 of movement of thelower region of said drum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N003,056,561 October 2, 1962 Risto Tapa ni Hukki It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, lines 63 to 65, the formula should appear as shown belowinstead of as in the patent:

C V D Signed and sealed this 25th day of June 1963,

( 5 EA L) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents

